DESCRIPTION: The proposed research continues an investigation of the neural mechanisms that govern the decision to ingest or reject the contents of the oral cavity. The neural substrates are located in the brainstem. Afferent activity (gustatory, thermal,tactile) that initiates and guides ingestion reaches the medulla and pons first. The response elicited by these stimuli can be switched from ingestion to rejection by vagal visceral activity that reaches the CNS first in the medulla. Likewise, the oral motor neurons and the interneurons projecting to them are found in the brainstem. The behavior of chronic precollicular decerebrate rats provides confirmation of the sufficiency of caudal brainstem mechanisms for ingestion and rejection responses. The proposed experiments use behavioral and electrophysiological techniques to test hypotheses about the function of the nucleus of the solitary tract (NST) and the parabrachial nucleus (PBN) in processing gustatory and visceral afferent information and examining the responses of oral motor interneurons during ingestion. There are three specific aims: First focal lesions will be used in CD rats to evaluate the contribution of the NST, PBN and the parvicellular reticular formation (RF) subjacent to the NST to simple and complex gustatory functions. CD rats with bilateral PBN lesions will be tested for responses to sucrose and for the effects of satiety. Rats with lesions of the anterior NST or parvicellular RF will be tested for preference - aversion functions, learned taste aversions and bodyweight regulation. Animals with lateral hypothalamic lesions will also be tested for the ability to acquire learned taste aversions and to express sodium appetite. For the second specific aim, electrophysiological studies are proposed in awake, behaving rats. Neural responses will be recorded in the NST and PBN for the effects of conditioned taste aversions and duodenal infusions of lipids, since it is clear that both experiences and immediate physiological conditioning can influence ingestion and rejection behavior. Medullary reticular formation neurons will be tested for responses to water, sapid stimuli and liquid diet. Oral motor interneurons in the RF that are found to respond differentially to the sensory characteristics of oral stimuli will be tested during more complex taste-guided behaviors. Experiments related to the third specific aim will examine the neural relationships among brainstem and fore brain areas implicated in the control of ingestive behavior. Neural responses will again be recorded in awake behaving animals. Electrical stimulation of the lateral hypothalamus, amygdala or bed nucleus of the stria terminalis will be used to alter the firing patterns of gustatory and oral motor interneurons in the brainstem.